Conventionally, a spark plug including a tubular insulator having a resistor incorporated therein is known (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2009-245716). In such a type of spark plug, a metal terminal is disposed at one end portion side of a through hole of the insulator, and a center electrode is disposed at the other end portion side of the through hole. The resistor is disposed between the metal terminal and the center electrode.
The resistor housed in the through hole of the insulator is formed from a mixture of glass powder and a conductive substance such as carbon black powder or metal powder. The content of metal in the resistor is not so high, and thus, in many cases, it is difficult to directly join the resistor to the metal terminal or the center electrode which are made of metal. Therefore, for example, a conductive glass seal layer containing an amount of metal powder larger than that in the resistor is disposed between the resistor and the metal terminal or between the resistor and the center electrode, thereby enhancing the joining force.
An example of a process for manufacturing such a spark plug including a resistor will be described below.                (1) After the center electrode is placed in the through hole of the insulator, conductive glass powder is packed therein, then raw material powder of a resistor composition is packed therein, conductive glass powder is further packed therein again, and the metal terminal is finally inserted thereinto to create an assembly.        (2) The assembly is brought into a heating furnace and heated to a temperature equal to or higher than the softening point of glass contained in the resistor composition and the conductive glass powder. Thus, the glass contained in the raw material powder of the resistor composition and the conductive glass powder melts.        (3) Thereafter, in a state where the glass melts, the metal terminal is squeezed-in in the axial direction of the metal terminal, and a state of supporting the metal terminal is maintained until the glass becomes solidified, whereby conductive glass seal layers are formed at the front side and the rear side of the resistor. As a result, the metal terminal and the center electrode are joined to the resistor via the respective conductive glass seal layers and also fixed to the insulator.        
In the spark plug disclosed in Japanese Patent Application Laid-Open (kokai) No. 2009-245716, even when the diameter of the conductive glass seal layer is equal to or less than 3.3 mm, since a joined surface of each conductive glass seal layer and the resistor is formed as a curved surface, occurrence of separation at the joined surface is suppressed.
Since the joined surface of each conductive glass seal layer and the resistor is formed as a curved surface, the joining strength therebetween can be enhanced. However, even in the case of exposure to a higher combustion pressure than in the conventional art, the airtightness between each conductive glass seal and the insulator is desirably sufficient. If the airtightness between the insulator and the conductive glass seal layer at the center electrode side is lost, a possibility arises that airtightness required for the spark plug cannot be maintained. There is a concern that such a decrease in sealability is likely to occur particularly when the diameter of the spark plug is reduced.
An advantage of the present application is a small-diameter spark plug which includes a resistor that is able to maintain sufficient airtightness.